Over the years, many different methods have been developed for removing liquids from solids. One common method involves a step by which the liquid is evaporated, leaving dry solids. In practice, such an evaporation step is usually preceded by at least one other dewatering step designed to remove as much liquid as possible before evaporation. The use of a pre-evaporation step or steps is preferred primarily for two reasons. First, the large amount of heat required to vaporize the liquid makes the evaporation step exceedingly expensive. Second, the time required to boil the liquid can make the evaporation step very time consuming. Thus, in order to reduce the cost and/or length of the evaporation step, a multi-step liquid removal process might be employed in which an evaporation step is preceded by any one or all of the following pre-evaporation steps: gravity settling; low-pressure filtration; and high-pressure filtration. Known techniques for accomplishing these individual steps are discussed in detail in "The Chemical Engineers Handbook".
When separating solids from water, as in the case of a primary water treatment system, gravity settling typically results in a slurry or sludge that is 1 to 10 percent solids by weight. The sludge can then be subjected to high-pressure filtration producing a filter cake that is approximately 30 to 40 percent solids by weight. The maximum percent solids that can be achieved by pressure filtration is limited by the point at which "solids hold-up" occurs (i.e., the point in the process at which the solid particles are compressed together to such an extent that they behave as a rigid block and will not compact further). However, when such a point in the process is reached, a significant amount of liquid still exists within the intersticies between the solid particles, the ratio of solids to liquid at this point being referred to as the "equilibrium solids concentration". In order to remove the remaining liquid, the filter cake is typically heated above the boiling temperature of the liquid and the liquid is evaporated off. The amount of energy required to remove the remaining liquid is still extremely large, even when such an evaporation step is preceded by a filtration step.
Most commercial filtration equipment, such as vacuum filters, leaf filters, centrifuges, belt filter presses and plate and frame filter presses, are incapable of producing filter cakes having an "equilibrium solids concentration", and, therefore, in practice, they usually produce what at best would be about a 40 percent solids filter cake. To completely dry a pound of such a filter cake, 0.6 pounds of liquid would have to be removed. If the liquid were water, at ideal efficiency almost 1000 BTU per pound would be required to evaporate the water, assuming such evaporation occurred at atmospheric pressure. Additional energy would also be required to raise the temperature of the solid/liquid mixture from ambient to vaporization temperature. Because the conventional drying processes are not 100 percent efficient, values of 1500-2000 BTU per pound of water removed are typical for such processes.
In view of the foregoing discussion, there is a real need for a process which is more effective than the known methods for removing liquids from solids. Because energy conservation is such an important concern nowadays, there is a further need for a deliquification process which, in addition to being more effective than the known methods, is also economic.